For example, Patent Document 1 discloses a fuel tank having a fuel tank body and a columnar member. Both ends of the columnar member are welded to fix opposed inner faces of the fuel tank body. The columnar member is, for example, in a cylindrical shape or an elliptic shape in a cross section. Members that are welded in the fuel tank body are, in addition to the columnar member, for example, a wave-dissipation plate and a bracket. The members to be welded on the inner faces of the fuel tank body are referred to as “welded members” hereinbelow.